### Ink in Water

One way to achieve this effect is using a looot of particles advected by a velocity field generated by some gas simulation. Why not use the gas sim itself ? Cause in order to achieve the same detail offered by particles, the resolution of the gas grid should be so high that even Hal 9000 wouldn't be able to handle it or Sky-net would suddenly become self-unaware.

So I generated a simple smoke sim using H13 Smoke Solver. I didn't use Pyro cause it was probably overkill for such a simple sim. Then I wedged and cached to disk 10 versions of the smoke sim varying parameters like the turbulence amplitude and twirl radius (finding out what parameters to wedge is an art on its own).

Once I had 10 different smoke sims, I had 10 different velocity field sequences that I can use to advect my points.
Now, two ways to advect points in Houdini are:

• create a POP system in a DOP Network and use the POP Advect by Volumes DOP node making sure to point it to the velocity fields cached on disk and re-imported somewhere else at the SOP level.
PROs: POP land provides a large range of nodes to control particle motion.
CONs: Slow
• convert the velocity to VDB, merge the fields into one single VDB vector field (with VDB Vector Merge - don't worry if Houdini complains cause the grid components are different), then use VDB Advect Points in a SOP Solver to advect your points.
PROs: Very fast.
CONs: you've to do everything yourself.
• PRO of this CONS: you CAN do whatever you want  :) !

When I mentioned thet the POP is slow, I still mean that it can process several hundreds of thousands of particles per second. The VDB solution can handle millions of points per second. Because of this, you can avoid rendering different versions of the particle sim and compositing them in order to smooth out the "particular" look (which is the main challenge of ink in the water effect).

So I decided to adopt the VDB solution.

In order to add extra points in the areas where the particles were more sparse, I added an Attribute Wrangle node in the end of the chain and I called it Fill Gaps. Since Attribute Wrangle works in the CVEX context, I am given the opportunity to add (or destroy) geometry.
The purpose of the script is basically adding points where there aren't.

This is the content of the script (make sure to create the proper UI parameters before sourcing the script) and feed the same point geometry both in input 1 and 2 of the Attrib Wrangle node.

Note: this fill gap algorithm is FAR from being perfect. Plus it just really fills the spaces between points larger than "mindist" within "seachrad". So the results might not be as expected. The good thing is that the added points remain surprisingly consistent during the animation, so , at least, you don't see crazy artifacts or points popping in suddenly, in the render.

float mindist=ch("mindist");
int maxpoints=ch("maxpoints");
int fillpoints=ch("fillpts");

vector clpos;
int i=0;
while(pciterate(handle))
{
if (i==0) // the first point found should be the closest, in this case, itself. We want to skip it.
{
i++;
continue;
}
pcimport(handle,"P",clpos);
if (length(@P-clpos)>mindist)
{
vector pointstep=(clpos-@P)/(fillpoints*2+1); // this ensures there are no duplicate point
// at the cost of doubling the fill points number
for (int t=0;t<fillpoints;t++)
}
}

After rendering 10 exr sequences, I imported them in Nuke and used them as sprites on a particle distribution scattered over a large plane, and rendered with DOF. Kinda slow actually, and the DOF in comp is always a PITA (has this improved in the last 15 years ? nope!).

This is the result.
Oh, there is a card disappearing in the last 10 frames, probably one of the Nuke particles ran out of fuel.

Ink in Water - test from Alessandro Pepe on Vimeo.

### Houdini - Explicit Cache

Classical Scenario:
You deadline is today at 7pm. Which means your deadline really is tomorrow around 9pm ! You've time to recalculate your huge flip sim that takes about 13 hours to complete (note, not 12 hours, not 14...13 the evil number). Each bgeo file is about 666mb. You hope you'll have enough HD space, but hey ... HD space is never enough ! Even Confucius knew this.
So you cross your fingers, and hit "Render" on your rop_geometry network to start your 999 frames FLIP sim.

Now usually, when you've only one chance to do it right and if you miss it you'll be fired, usually this is what happens in order:
1 - you run out of HD space on the frame 998.
2 - Linux crashes for the first time in 12 years
3 - Houdini crashes for the 13th time in 2 hours
4 - power outage in the whole <city where you are in that moment>

You better remember that one of these 4 things (if not all of them) will happen. It's important to be positive.

How will our fellow FX Artist save his job, and manage to pay the rent of his mansion with pool in the Hollywood Hills ?

The answer is this gorgeous little gem of pure love called "explicit cache" and his little friend "explicit frames to cache" on the DOP Network node.

This option is off by default, cause the files generated can fill your 12Tb HD very quickly.
But this is where the second option comes really in handy !
You don't have to save ALL the .sim files for each one of your 999 frames of simulation.
You can save the last, say, 5 (or less maybe) and restart the sim from the smallest frame number in your cache ! This way you'll not clog your HD with unnecessary GIGANTIC .sim files, and you'll still be able to resume your sim.

Example:
n.4 happened. And I managed to simulate only 362 frames of my 999 frames sim. But I was wise enough to enable the explicit cache option and specify a frame history of 5.
So, if I go in my ...../simcache directory I'll see these 5 files:

What I usually do is delete the last one. Why ? Well ... I love Houdini, but I will never trust that he managed to write out the last sim file while the power of my computer was off. And you've a pretty good clue of this checking the file size of the files. They are all ~153 Mb, apart from the last one ! MM ... suspicious. Delete !

Perfect. Now you have 4 cache files and you're sure they are fully functional.

Now you can restart your sim starting from the frame 358 and Houdini will seamlessly continue simming like if you started from frame 1.

This saved my <censored> several times already.

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